Mass separation



Dec. 23, 1958 c. F. ROBINSON I 2,866,097

MASS SEPARATION Filed April so, 1956 3Sheets-Sheet 1 lNLET 70 EXHAUST SEE FIG. 2.

R. F OSClLLATO/ 3 'C. SUPPLY 5 FIG. 2.

INVEN TOR. CHARLES E ROBINSON @M Ma M v A TTORNEVS 1958 c. F. ROBINSON 2,866,097

MASS SEPARATION Filed April so, 1956 3 Shets-Sheet 2 P76. 3. q 3 PARABOL/C POTENT/AL Q! a ACTUAL it POTENT/AL 8 3 COLLECTOR RE SON TAN T ION COLLE C TOR NON- RESONTANT I /0/\/ COLLECTOR] POTENT/AL XAX/S VAX/S IN VEN TOR. CHA RL E S F ROBINSON A TTORNEKS 1958 c. F. RbslNsoN 2, 866,097

MASS SEPARATION Filed April 30, 1956 5' Sheets-Sheet 3 g 5% $3 c gwfi E E f'\./ 3 Q) t 5 INVENTOR. Q 3 02 CHARLES E ROBINSON k 31, BY

m i M A TTORNEVS 'ions in resonance with the R. F.

United States Patent F MASS SEPARATION Charles F. Robinson, Pasadena, Calif., assignor to Consolidated Electrodynamics Corporation, Califi, a corporation of California Application April 30, 1956, Serial No. 581,414

5 Claims. (Cl. 250-419) Pasadena,

cording to specific mass under the influence of suitably shaped electrical and magnetic fields, and selective sensing of one or more specific masses.

A comparatively recent development in this art which may be termed D. C. (direct current) resonance mass spectrometry, makes use of characteristic ion oscillation in a D. C. field of parabolic configuration. In co-pending United States patent application Serial No. 252,044, filed October 19, 1951 by Clifford E. Berry and Charles F. Robinson, now U. S. Patent 2,688,088, one method of D. C. resonance mass separation is disclosed in which ions of a given specific mass are sensed through the inductive loading of a tuned circuit sensible to ion oscillation in the parabolic D. C. field and tuned to the resonant frequency of the specific mass in resonance. This method is characterized by independence of any superimposed R. F.'(radio frequency) stimulus and its adaptability to ion injection into the D. C. field from an external source.

Another form of D. C. resonance mass separation, as described in United States Patent No. 2,570,158, issued to P. O. Schissel on October 2, 1951, contemplates collection of ions of a specific mass. This is accomplished by disposing collector electrodes adjacent the opposite boundaries of the D. C. field defined by the potential maxima of the field, and superimposing an R. F. driving field across a part or the whole of the D. C. field whereby field will acquire sufficient energy to escape from the D. C. field and strike one or the other of the collector electrodes. It is this form of D. C. resonance mass separation to which the present invention relates and the following discussion and description are so directed.

The prior art has totally failed to take into account the effects of electrons in this type of system. Since at least one electron is liberated in the formation of each ion, and inasmuch as such electrons are accelerated directly into the collector electrodes, spurious discharges inevitablyresult unless means are provided to counteract this effect. In some circumstances the spurious signals originating from electron discharge at the collector electrode may be of, sufficient magnitude to completely mask positive ion discharge.

Another previously unresolved limitation inherent in 2,866,097 Patented Dec. 23, 1958- 2 this procedure is the indiscriminate phasing of the ions formed within the confines of the .field. lons formed at an inopportune phase angle require an inordinate number of cycles of oscillation to reach a collector electrode. Such ions contribute very little to the resolving power and their space charge effects are out of all proportlon to the amount of discharge current produced.

A third difiiculty is in the matter of ionization within the shaped field. To attain a suitable sensitivity an axial magnetic field has heretofore generally been employed to collimate the ions to limit migration other than on the ordinate axis of the parabolic field. Such a magnetic field makes it practically impossible to develop an ionizing electron beam normal to the field axis unless special techniques are employed.

The present invention is directed to improvements in methods and apparatus for D. C. resonance mass separation to minimize or remove the several limiting conditions outlined above.

In one aspect the invention contemplates a method of mass separation comprising forming ions in a parabolic D. C. field by means of an ionizing electron beam, superimposing an alternating electrical field on the D. C. field to expel from the D. C. field those ions which are in resonance with the alternating field, collecting the expelled ions exteriorly of the D. C. field while separating electrons from the resonant ions between the point of ion formation in the ionizing electron beam and the point of collection of expelled ions.

In another aspect the invention contemplates a mass spectrometer comprising an analyzer chamber, a plurality of more or less planar electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic D. C. field across a space in the chamber, means for imposing an R. F. electrical. field on the D. C. field, ion collection means including at least one collector electrode disposed adjacent a boundary of the D. C. field, means developing an ionizing electron beam directed transversely of the D. C. field and means adapted to limit electron travel in the directionof the ion collecting means. In a preferred embodiment, means are provided for preventing random exit of ions from the field. If an axial magnetic field is employed for this purpose, additional provision is made for permitting the ionizing electron beam to travel transversely of such a magnetic field.

The invention will be more clearly understood in all of its embodiments from the following detailed description thereof taken in relation to the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional elevation through a mass spectrometer in accordance with the invention;

Fig. 2 is a transverse sectional elevation taken on the line 22 of Fig. 1;

Fig. 3 is a graph of the direct current electrical potential established across the region of ion oscillation as accomplishing one means of separation of electrons and resonant ions;

Fig. 4 shows the momentary direct current field distribution periodically developed as a means for maintaining sharp resolution and at the same time separating electrons from resonant ions; and

Fig. 5 is a schematic diagram of an electrical circuit for achieving the field conditions depicted in Fig. 4.

The mass spectrometer as shown in longitudinal sectional elevation in Fig. 1 and in transverse sectional elevation in Fig. 2 comprises an envelope 10 provided with an exhaust line 11 for connection to an evacuating system (not shown) and a sample inlet line 12 for introducing a sample to be analyzed into the instrument. A plurality of field-shaping electrodes 14, 15, 16, 17, 18, 19, 20, 21,

22 are arranged within the envelope in spaced parallel relation. These electrodes may be in the form of grids, as illustrated, annular rings or any generally planar form to permit ion oscillation in the region defined by the electrodes. A pair of collector electrodes 24, 25 are disposed in the envelope respectively adjacent the outer field-shaping electrodes 14 and 22, i. e. at opposite ends of the ordinate axis of the shaped field, the collector electrodes being adapted to collect and discharge ions expelled from the region of the field-shaping electrodes in the manner hereinafter described. The two collector electrodes 24, 25 are interconnected in circuit with an amplification and sensing circuit 26, shown in block diagram, and through a dropping resistor 28 to ground. The amplifying and sensing circuit 26 and associated resistor 28 may take any suitable form, many such systems being well known in the art.

The field-shaping electrodes are interconnected in pairs, the pairing being between correspondingly positioned electrodes on opposite sides of the median plane, as defined by the central electrode 18. The several electrodes 14, 15, 16, 1'7 and the median electrode 18, together with their respective corresponding paired electrodes 22, 21, 20, and 19, are connected across a voltage divider 30. The opposite sides of the voltage divider 30 are connected across a direct current supply 32 and inductively across a radio frequency oscillator 34.

For reasons hereinafter made apparent, the electrode system is biased by means of a battery 36 so that the col lector electrodes 24, 25 are at a negative potential with respect to the field-forming electrode array.

One of the problems encountered in the operation of a mass spectrometer of this type is that of holding the oscillating ions within the shaped field as developed and defined by the electrode array. One means of overcoming this problem is by the establishment of an axial magnetic field. This is accomplished in the embodiment of Fig. l by means of an electromagnetic coil 38 disposed coaxially around the envelope 10 and energized from a voltage source 39, illustrated as a battery.

The development of such a coaxial magnetic field to collimate the oscillating ions gives rise to a further problem with respect to the development of an ionizing electron beam.

As shown in the drawing, an electron gun 4t) comprising an emitter 41 and a shield electrode 42 is disposed in the envelope 10 to direct an electron beam 43 transversely of the shaped field and at the median plane thereof toward an electron target 44. Refinements of differential pumping, electron gun construction, and the like, are well known in the art and form no part of the present invention. Moreover, there is no requirement that the electron beam be located at the median plane of the field although such orientation is presently preferred. An electron beam in the presence of an axial magnetic field of sufiicient magnitude to accomplish the collimation referred to above will be unable to traverse the region between the electron gun and target electrode at any practical operating voltage, the magnetic field having the efifect of turning the electron beam about itself and driving it back to the emitter.

I have now devised a means of overcoming the effects of the axial magnetic field as related to the electron beam, which means also has the further advantageous effect of permitting modulation of the beam so as to accomplish desired phasing of ion formation. This means is illustrated in Figs. 1 and 2 with the electrical connections involved being more clearly evidenced in the transverse sectional elevation of Fig. 2. A pair of plate electrodes 46, 47 are disposed adjacent the median plane of the shaped field and are arranged parallel to each other on opposite sides of the beam. The electron emitter 41 is connected across a voltage source 50, the shield electrode 42 being biased positively with respect to the emitter by a. voltage source 52 to develop a propelling potential for directing the emitted electrons toward the target electrode 44. The plate electrode 47 is held at a high negative potential with respect to the potential of the emitter 41 and the plate electrode 45 is connected across a source 54 of square wave potential.

At any instant at which the potential on the plates 46, 47 are the same, the potential therebetween is negative with respect to the source of electrons, and no electrons can traverse the gap defined by the plates. In the moment in which electrode 46 is driven positive with respect to electrode 47, responsive to the square wave potential applied thereto, then the space potential intermediate the electrodes 46, 47 can be made positive with respect to the electron source, and electrons can traverse the gap to the target 44. This arrangement accomplishes two purposes. First, it provides a very strong electrostatic field transverse to the electron beam which can be used to buck out or obscure the efiect of the axial magnetic collimating field. By way of example of the parameters involved: With an axial magnetic field of 1000 gauss, an electrostatic field ofapproximately 6000 volts per centimeter will provide a transverse force sufiicient to just cancel the transverse magnetic force on an electron beam of volts energy and thus to permit such an electron beam to travel in a straight line to target 44. Secondly, the resultant modulation of the electron beam correspondingly modulates the ion beam, and by proper frequency control of the square wave modulating potential the ion beam is self-chopped or phased to avoid the space charge problem induced by out-of-phase particles.

In considering the operation of the described instrument, reference is made to a particle of mass m (in grams) carrying a charge q (e. s. u.) placed in a region where the electrical potential (statvolt) along the X axis (ordinate axis of the parabolic field) is symmetrical with respect to the origin or median plane (X :0). For the purposes of mass resolution in accordance with the present invention it is desired to develop a potential distribution in which the period of oscillation of the particle will be amplitude independent. It can be shown mathematically that a parabolic potential distribution represented by the function:

V=AX (1) where V=electrical potential at a point X, A=a numerical constant, and X =distance from potential minimum,

is the most general distribution which will yield strictly simple harmonic motion of the particle. If an A. C. potential of frequency w is superimposed on the parabolic D. C. potential with a completely arbitrary distribution along the X axis, it can be shown that the component of the A. C. potential, which is in symmetry with the median plane, is effective only in accelerating even harmonic particles of the resonant mass and the unsymmetric components are effective for accelerating fundamental and odd harmonic particles. Hence, it is possible to completely exclude one class or another by shaping the R. F. potential either symmetrically or anti-symmetrically with respect to the median plane of the X axis. A representative expression can be developed mathematically descriptive of the A. C. field distribution required to eliminate harmonics, one such field being defined by the function V=A X sin (2w p) where A =a numerical constant,

X =space coordinate of the field,

w=the natural resonant frequency of the particle in the D. C. field V=AX tp=a constant phase angle.

The particular A. C. field distribution defined by Equation 2 avoids all harmonic interference and may be developed in the system as illustrated in Figs. 1 and 2.

As shown in Fig. 1 theR. F. oscillator 34, by means of which the A. C. field is developed acrossthe electrode array, is connected to the electrodes in the same pattern as the D. C. supply 32 with aconsequent similar symmetrical distribution of the A. C. field along the axis. By symmetrical distribution is meant that the potential at two points equally spaced on opposite sides of the median plane is approximately the same and not necessarily that the potential gradient between the median plane and the extremities of the axis are uniform. In fact, in a parabolic distribution, as contemplated, uniformity does not exist but the potential field is symmetrical within the above definition.

As previously mentioned, one of the principal problems encountered in R. F. mass separation of the type herein contemplated is the migration of electrons toward the collector electrodes. One electron is developed upon the formation of each ion and under normal circumstances such electrons are accelerated with considerable energy in the direction of the collector electrodes. The present invention contemplates several alternative means and methods for preventing actual impingement of such vagrant electrons on the collector electrode. One such means is shown in Fig. l and is illustrated diagrammatically in Fig. 3. bolic D. C. field is represented by curve A plotted with respect to the conventional X and Y axes, the Y axis representing increase in potentialaway from the origin. The collector is illustrated with respect to its potential relationship to the shaped field of curve A and, as illus trated, is maintained at a negative potential with respect to any part of the shaped field. This is accomplished in the apparatus of Fig. 1 by means of the bias battery 36 which imposes a positive bias on all of the electrodes of the field-shaping array. As a consequence, electrons formed anywhere in the field will have insufficient energy to overcome the negative bias of the collector electrodes and will be repelled therefrom. This method of eliminating vagrant electrons is entirely satisfactory when operating the instrument at comparatively low resolution. The use of a small negative bias on mass spectrometer collector electrodes to insure ion discharge thereon is old in the art (cf. Schissel U. S. 2,570,158). However, the bias contemplated herein is of a different order of magnitude, i. e. negative with respect to every part of the system and for a different purpose, i. e. to prevent discharge of electrons whatever their energy may be.

At high resolution under which conditions resonant particles oscillate through a great many cycles within the shaped field, a phase error is introduced as a consequence of particle entry into a so-called region of detuning indicated by shaded portions on the diagram of Fig. 3. Each time a resonant particle enters this region of detuning a slight phase shift is induced and at high resolution the number of entries are such as to accumulate a large enough phase error that such a resonant particle will no longer gain energy from the A. C. field and will never reach the collector at all.

To overcome the effects of such phase shift the instrument may be operated to maintain the potential relationships as illustrated in Fig. 4 wherein the resonant ion collector is again shown as being at a negative potential with relation to the rest of the system. In Fig. 4 the shaped field is represented by the parabolic curve B such that non-resonant ions are limited in their oscillation to an amplitude at and resonant ions to an amplitude b. By pulsing the field-shaping electrodes lying in the region marked 0, resonant ions oscillating in this region can be periodically dumped to the collector electrode at the same time non-resonant ions can be dumped to a non-resonant ion collector by the pulsation of the field-shaping electrodes in the region marked a as illustrated. This constitutes one means of minimizing space charge problems. In other words, the conditions ob tained in Fig. 3 may be developed only periodically in the Referring to Fig. 3, the parasituation depicted in Fig. 4 so that the phase shift'problem is not encountered. 'Many means for accomplishing this pulsation will be obvious to anyone skilled in the art.

One suchmeans is shown in Fig. 5 which is a schematic circuit diagram showing the electrode array of Fig. l and the circuitry modifications necessary to accomplish the voltage distribution depicted in' Fig. 4. In this circuit, field forming electrodes 60 through 68 are connected to a D. C. supply 69 through a suitable voltage divider network 70 to produce the parabolic D. C. potential shown by curve B in Fig. 4. As in the embodiment of Fig. 1, an R. F. oscillator 72 is connected across the electrode array to produce the R. F. field required for operation of the device. To develop the periodic dumping potential shown in the dotted line curve of Fig. 4, a dumping pulse oscillator 74 producing periodic negative pulses as shown in the output train 75 is connected across a voltage divider 76. The several field forming electrodes are connected to appropriate points on this voltage divider,

each through a capacitor C and a resistor R, the resistors R not necessarily of the same value. A so-called resonant ion collector electrode 78 is connected to an amplifying and sensing system 79, anda socalled non-resonant ion dumping collector 80 may, if desired, be connected to a second amplifying and sensing system 81.

Each negative pulse produced by the dumping pulse oscillator is distributed among the electrodes by the voltage divider network 76 to produce the potential distribution shown at'the dotted line of Fig. 4 and to accomplish the consequences described in relation to that figure.

In any of the embodiments of the invention the sample spectrum may be scanned by varying the frequency of the A. C. field to bring ions of differing mass into resonance therewith or by varying the amplitude of the D. C. field as desired.

Other forms of apparatus or electrical circuitry may be derived for effectuating ion collimation and electron control within the scope of the invention which teaches methods and apparatus for improving the effectiveness of D. C. resonance mass spectrometry.

I claim:

1. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic direct current field across a space in the chamber, means for superimposing a radio frequency field on the direct current field, ion collection means including at least one collector electrode disposed exteriorly of and adjacent a boundary of the direct current field, means' developing an ionizing electron beam directed transversely of the direct current field, means biasing the plurality of electrodes positively with respect to the collecting means, and means periodically pulsing some of the electrodes including the boundary electrode to dump resonant ions from the field to the collector electrode.

2. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic direct current field across a space in the chamber, means for superimposing a radio frequency field on the direct current field, ion collection means including at least one collector electrode disposed exteriorly of and adjacent a boundary of the direct current field, means developing an ionizing electron beam directed transversely of the direct current field, means biasing the plurality of electrodes positively with respect to the collecting means so that the collecting means is at a negative potential with respect to every part of the shaped field, and means periodically pulsing some of the electrodes including the boundary electrode to dump resonant ions from the field to the collector electrode.

3. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic direct current field across a space in the chamber, means for superimposing a radio frequency field on the direct current field, ion collection means including a separate collector electrode disposed exteriorly of and adjacent the extremities of the parabolic direct current field, means developing an ionizing electron beam directed transversely of the direct current field, means biasing said plurality of electrodes positively with respect to at least one of said collecting means so that said one collecting means is at a negative potential with respect to every part of the shaped field, means periodically pulsing some of the electrodes including the boundary electrode adja cent said one collector electrode, and means periodically pulsing others of the electrodes to dump non-resonant ions from the field to the other collector electrode.

4. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic direct current field across a space in the chamber, means for superimposing a radio frequency field on the direct current field, ion collection means including at least one collector electrode disposed exteriorly of and adjacent a boundary of the direct current field, means developing an ionizing electron beam directed transversely of the direct current field, means developing a magnetic field across the chamber transversely of the longitudinal 3 axis of the parabolic direct current field and in the region between the boundary of the direct current field and the respective adjacent collector electrode, and means periodically pulsing some of the electrodes including the boundary electrode to dump resonant ions from the field to the collector electrode.

5. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including the electrodes for developing a substantially parabolic direct current field across a space in the chamber, means for superimposing a radio frequency field on the direct current field, ion collection means including a separate collector electrode disposed exteriorly of and adjacent the extremities of the parabolic direct current field, means developing an ionizing electron beam directed transversely of the direct current field, means developing a magnetic field across the chamber transversely of the longitudinal axis of the parabolic direct current field and in the region between the said one of the collecting means and the adjacent boundary of the direct current field, means periodically pulsing some of the electrodes including the boundary electrode to dump resonant ions from the field to one collector electrode, and means periodically pulsing others of the electrodes to dump" non-resonant ions from the field to the other collector electrode.

No references cited. 

